Beam Diagnostics Instruments Uli Raich CERN BE - BI (Beam Instrumentation)

Overview First part: Introduction Overview of measurement instruments TV Screens Scintillating screens Faraday Cup Beam Current Transformer Beam Position Monitor Profile Detectors SEMGrids Wire Scanners Beam Loss Monitors Second part Some depicted examples of beam parameter measurements

Introduction An accelerator can never be better than the instruments measuring its performance!

Linac-4 layout Commissioned up to 100 MeV (CCDTL) 45 keV 3 MeV 12 MeV 50 MeV 100 MeV 160 MeV H-Source RFQ DTL CCDTL PIMS LEBT Chopper Line Transfer Line to PSB Commissioned up to 100 MeV (CCDTL) The LEBT instrumentation is temporarily extended with a slit/grid emittance meter MEBT line transformer and wire scanners show chopper performance Temporary 3/12 MeV measurement line Temporary 50/100 MeV measurement line Beam characterization with operational diagnostics at 160 MeV

Diagnostic devices and quantity measured Instrument Physical Effect Measured Quantity Effect on beam Faraday Cup Charge collection Intensity Destructive Current Transformer Magnetic field Non destructive Wall current monitor Image Current Longitudinal beam shape Pick-up Electric/magnetic field Position Secondary emission monitor Secondary electron emission Transverse size/shape, emittance Disturbing, can be destructive at low energies Wire Scanner Secondary particle creation Transverse size/shape Slightly disturbing Scintillator screen Atomic excitation with light emission Transverse size/shape (position) Residual Gas monitor Ionization

A beam parameter measurement

Scintillating Screens Method already applied in cosmic ray experiments Very simple Very convincing Needed: Scintillating Material TV camera In/out mechanism Problems: Radiation resistance of TV camera Heating of screen (absorption of beam energy) Evacuation of electric charges

Interaction of particles with matter Coulomb interaction Average force in s-direction=0 Average force in transverse direction <> 0 Mostly large impact parameter => low energy of ejected electron Electron mostly ejected transversely to the particle motion

High energy loss a low energies Heavy ions at low energy are stopped within a few micro-meters All energy is deposited in a very small volume

Screen mechanism Screen with graticule

Results from TV Frame grabber First full turn as seen by the BTV 10/9/2008 Uncaptured beam sweeps through he dump line For further evaluation the video signal is digitized, read-out and treated by program

Layout of a Faraday Cup Electrode: 1 mm stainless steel Only low energy particles can be measured Very low intensities (down to 1 pA) can be measured Creation of secondary electrons of low energy (below 20 eV) Repelling electrode with some 100 V polarisation voltage pushes secondary electrons back onto the electrode

Faraday Cup

Electro-static Field in Faraday Cup In order to keep secondary electrons within the cup a repelling voltage is applied to the polarization electrode Since the electrons have energies of less than 20 eV some 100V repelling voltage is sufficient

Energy of secondary emission electrons With increasing repelling voltage the electrons do not escape the Faraday Cup any more and the current measured stays stable. At 40V and above no decrease in the Cup current is observed any more

Current Transformers Magnetic field Fields are very low ri Capture magnetic field lines with cores of high relative permeability (CoFe based amorphous alloy Vitrovac: μr= 105) ri ro Beam current

The active AC transformer The ideal transformer The active AC transformer The AC transformer RF RL CS A Ls RL CS R Inductance L of the winding Transformer output signal Beam signal

Principle of a fast current transformer 500MHz Bandwidth Low droop (< 0.2%/ms) Image Current Ceramic Gap 80nm Ti Coating 20W to improve impedance BEAM Calibration winding Diagram by H. Jakob

Fast current transformers for the LHC U. Raich CERN BE/BI African School on Fundamental Physics

Magnetic shielding Transformer steel (μ2) Soft iron (μ1) Permalloy (μ3) Shield should extend along the vacuum chamber length > diameter of opening Shield should be symmetrical to the beam axis Air gaps must be avoided especially along the beam axis Shield should have highest μ possible but should not saturate

Calibration of AC current transformers The transformer is calibrated with a very precise current source The calibration signal is injected into a separate calibration winding A calibration procedure executed before the running period A calibration pulse before the beam pulse measured with the beam signal

Measuring Beam Position – The Principle Slide by R. Jones + - - + - - + + - - + - - + + - - + - + - - + + - - - + - - + + - + - - + - + + + + - + + - - - - + - - + - + - - + + - + - + - + - - + + - + + - + - - + + - + + - + - - + + - + +

Wall Current Monitor – The Principle V + - - + - - + - + + - - + - + - - + - - + - Ceramic Insert - + - - + + + + + + - + - - + - + - - - + - - + - - + + - + + - + - - + + - + +

Electrostatic Monitor – The Principle V + - - + - + - + + - - + - + - - + - + - - - - + - - + + - - + + - + - - + - + - - + - - + - - + + - - + - + - - - - - + - + + - - + + - - + - - + - + - + + - + - + + - + + - + - - + - + +

Position measurements w U d U d If the beam is much smaller than w, all field lines are captured and U is a linear function with replacement else: Linear cut (projection to measurement plane must be linear)

Shoebox pick-up UL UR Linear cut through a shoebox

Doubly cut shoebox Can measure horizontal and vertical position at once Has 4 electrodes

Simulatenous horizontal and vertical measurement b d a c b c a d

Profile measurements Secondary emission grids (SEMgrids) When the beam passes secondary electrons are ejected from the wires The current flowing back onto the wires is measured The ejected electrons are taken away by polarization voltage

Profiles from SEMgrids Charge density projected to x or y axis is measured One amplifier/ADC per wire Large dynamic range Resolution is given by wire distance Used only in transfer lines U. Raich CERN BE/BI African School on Fundamental Physics

Slow Wire scanners 33 mm Carbon wires mounted in L-shape on the same fork support and scanning the beam at 45 degrees (one scanning position per pulse) Time resolution of 4 ms within the beam pulse (250 kHz ADC)

Fast Wire Scanners A thin wire is quickly moved across the beam Secondary particle shower is detected outside the vacuum chamber on a scintillator/photo-multiplier assembly Position and photo-multiplier signal are recorded simultaneously U. Raich CERN BE/BI African School on Fundamental Physics

Wire scanner profile High speed needed because of heating. Adiabatic damping Current increase due to particle speed increase Wire speeds of up to 20m/s => 200g acceleration U. Raich CERN BE/BI African School on Fundamental Physics

Stored Beam Energies Quench Levels Units Tevatron RHIC HERA LHC (Based on graph from R. Schmidt) LHC extrapolation Energy in beam 7 times Tevatron Intensity of beam 30 times Tevatron Number of LHC of moving elements in the LHC about 200 Energy stored in both beams: 0.7 GJoule heat up from cryogenic temperature and melt 0.1 m3 of Copper Quench Levels Units Tevatron RHIC HERA LHC Instant loss (0.01 - 10 ms) [J/cm3] 4.5 10-03 1.8 10-02 2.1 10-03 - 6.6 10-03 8.7 10-04 Steady loss (> 100 s) [W/cm3] 7.5 10-02 5.3 10-03

Beam power in the LHC The Linac beam (160 mA, 200μs, 50 MeV, 1Hz) is enough to burn a hole into the vacuum chamber What about the LHC beam: 2808 bunches of 15*1011 particles at 7 TeV? 1 bunch corresponds to a 5 kg bullet at 800 km/h

Beam Dammage Fermi Lab‘sTevatron has 200 times less beam power than LHC!

Beam Loss Monitor Types Design criteria: Signal speed and robustness Dynamic range (> 109) limited by leakage current through insulator ceramics (lower) and saturation due to space charge (upper limit). Ionization chamber: N2 gas filling at 100 mbar over-pressure Length 50 cm Sensitive volume 1.5 l Ion collection time 85 s Both monitors: Parallel electrodes (Al, SEM: Ti) separated by 0.5 cm Low pass filter at the HV input Voltage 1.5 kV Secondary Emission Monitor (SEM): Length 10 cm P < 10-7 bar ~ 30000 times smaller gain Chamber design considerations: Signal speed and robustness against ageing (and loss of chamber gas) BLMB: Planned to use standard ACEM detectors (or photo-multipliers), and acquisition systems of LHC BCT. Not part of baseline scenario, for refined beam observation.

Quench levels

Industrial production of chambers Beam loss must be measured all around the ring => 4000 sensors! U. Raich CERN BE/BI African School on Fundamental Physics

System layout

Successive running sums